Id get a DIMM out and start inspecting your grounds. I had the same issue in my z and my 91 accord, turned out to be poor grounds. At idle it should be in the 13's. if anything you can go to your local autoparts store and they can test your alternator in car.
You can put a full load on your electrical system and get the DIMM and test from the battery ground point to the frame. It should be little if not none, however if its excessive then there is the issue. ALSO you can do this with the power, you can go from the power positive of the battery to the output terminal on the alternator.
My van last year would idle around 12.8 which was not acceptable, and didnt get over 13.6 at throttle. changed the alternator, not much change, then Cleaned up the grounds and added an additional ground (just one) and boom, idles at 13.7 and with throttle 14.4 volts. With a load just over 13.9v